Liquid ink printing may take a number of forms. In ink jet printing, exemplified by U.S. Pat. No. 4,544,931 (Watanabe et al.), a liquid droplet is ejected from a single scanning nozzle and in U.S. Pat. No. 4,593,295 (Matsufuji et al.), liquid droplets are ejected from multi-nozzle, multi-color heads arranged for scanning. In electroosmotic ink recording, exemplified by U.S. Pat. No. 4,383,265 (Kohashi), ink droplets are made to fly from the tip of a needle shaped recording electrode. Similarly, in electrostatic ink ejection, exemplified by U.S. Pat. No. 4,166,277 (Cielo et al.), ink is retained in holes of an ink reservoir and is attracted out of the holes by the selective application of a voltage between the ink and selected electrodes. In acoustic ink printing, exemplified by U.S. Pat. No. 4,308,547 (Lovelady et al.), a liquid drop emitter focusses acoustic energy to eject a liquid ink.
In most applications, an ejected droplet must be deposited upon a receiving medium in a predetermined, possibly controlled, fashion. For example, when color printing it is very important that an ejected droplet accurately mark the recording medium in a predetermined fashion so as to produce the desired visual effect. The need for accurate positioning of ejected droplets on a receiving medium makes it desirable to eject droplets of the different colors in the same pass of the printhead across the recording medium, otherwise slight variations between the relative positions of the droplet ejectors and the receiving medium, or changes in either of their characteristics or the characteristics of the path between them, can cause registration problems (misaligned droplets).
Acoustic ink printing provides a mechanism for depositing very small ejected droplets in an accurate manner. When using acoustic ejection for color printing where more than one material is being ejected, it is beneficial to use a material deposition head with multiple ejector units. By material ejection head, it is meant a structure capable of ejecting a selected material from an associated chamber which is either the only chamber, or is one that is isolated from the other chambers. Therefore, a material deposition head with multiple ejector units is a structure capable of ejecting multiple materials. In terms of color printing, a material deposition head with multiple ejector units is a printhead capable of holding and ejecting more than one color of ink.
More detailed descriptions of acoustic droplet ejection and acoustic printing in general are found in the following U.S. patents and in their citations: U.S. Pat. No. 4,308,547 by Lovelady et al, entitled "LIQUID DROP EMITTER," issued Dec. 29, 1981; U.S. Pat. No. 4,697,195 by Quate et al., entitled "NOZZLELESS LIQUID DROPLET EJECTORS," issued Sept. 29, 1987; U.S. Pat. No. 4,719,476 by Elrod et al., entitled "SPATIALLY ADDRESSING CAPILLARLY WAVE DROPLET EJECTORS AND THE LIKE," issued Jan. 12, 1988; U.S. Pat. No. 4,719,480 by Elrod et al., entitled "SPATIALS STABILIZATION OF STANDING CAPILLARY SURFACE WAVES," issued Jan. 12, 1988; U.S. Pat. No. 4,748,461 by Elrod, entitled "CAPILLARY WAVE CONTROLLERS FOR NOZZLELESS DROPLET EJECTORS," issued May 31, 1988; U.S. Pat. No. 4,751,529 by Elrod et al., entitled "MICROLENSES FOR ACOUSTIC PRINTING," issued Jun. 14, 1988; U.S. Pat. No. 4,751,530 by Elrod et al., entitled "ACOUSTIC LENS ARRAYS FOR INK PRINTING," issued Jun. 14, 1988; U.S. Pat. No. 4,751,534 by Elrod et al., entitled "PLANARIZED PRINTHEADS FOR ACOUSTIC PRINTING," issued Jun. 14, 1988; U.S. Pat. No. 4,959,674 by Khri-Yakub et al., entitled "ACOUSTIC INK PRINTHEAD HAVING REFLECTION COATING FOR IMPROVED INK DROP EJECTION CONTROL," issued Sept. 25, 1990; U.S. Pat. No. 5,028,937 by Khuri-Yakub et al., entitled "PERFORATED MEMBRANES FOR LIQUID CONTROL IN ACOUSTIC INK PRINTING," issued Jul. 2, 1991; U.S. Pat. No. 5,041,849 by Quate et al., entitled "MULTI-DISCRETE-PHASE FRESNEL ACOUSTIC LENSES AND THEIR APPLICATION TO ACOUSTIC INK PRINTING," issued Aug. 20, 1991; U.S. Pat. No. 5,087,931 by Rawson, entitled "PRESSURE-EQUALIZED INK TRANSPORT SYSTEM FOR ACOUSTIC INK PRINTERS," issued Feb. 11, 1992; U.S. Pat. No. 5,111,220 by Hadimioglu et al., entitled "FABRICATION OF INTEGRATED ACOUSTIC INK PRINTHEAD WITH LIQUID LEVEL CONTROL AND DEVICE THEREOF," issued May 5, 1992; U.S. Pat. No. 5,121,141 by Hadimioglu et al, entitled "ACOUSTIC INK PRINTHEAD WITH INTEGRATED LIQUID LEVEL CONTROL LAYER," issued Jun. 9, 1992; U.S. Pat. No. 5,122,818 by Elrod et al., entitled "ACOUSTIC INK PRINTERS HAVING REDUCED FOCUSING SENSITIVITY," issued Jun. 16, 1992; U.S. Pat. No. 5,142,307 by Elrod et al., entitled "VARIABLE ORIFICE CAPILLARY WAVE PRINTER," issued Aug. 25, 1992; and U.S. Pat. No. 5,216,451 by Rawson et al., entitled "SURFACE RIPPLE WAVE DIFFUSION IN APERTURED FREE INK SURFACE LEVEL CONTROLLERS FOR ACOUSTIC INK PRINTERS," issued Jun. 1, 1993. All of these patents are hereby incorporated by reference.
The standard acoustic ink print head embodies a substrate having an acoustic wave generating means which is generally a planar transducer used for generating acoustic waves of one or more predetermined wave lengths. The wave generating means is positioned on the lower surface of the substrate. The transducer noted above is typically composed of a piezoelectric film such as zinc oxide positioned between a pair of metal electrodes, such as gold electrodes. Other suitable transducer compositions can be used provided that the unit is capable of generating plane waves in response to a modulated RF voltage applied across the electrodes. The transducer will be generally in mechanical communication with the substrate in order to allow efficient transmission of the generated acoustic waves into the substrate.
Generally an acoustic lens is formed in the upper surface of the substrate which is used for focusing acoustic waves incident on its substrate side to a point of focus on its opposite side. The acoustic lenses (whether spherical lenses or Fresnel lenses) are generally adjacent to a liquid ink pool which is acoustically coupled to the substrate and the acoustic lens. By positioning the focus point of such a lens at or very near a free surface of the liquid ink pool, droplets of ink can be ejected from the pool.
In the past to achieve varying color levels in acoustic ink printing, three approaches have been identified:
In the first approach, changing the length of the RF (and hence the acoustic burst) increases the droplet size by up to two times from its diffraction-limited minimum diameter of approximately one wave length. The second approach is to vary the number of droplets that are deposited per pixel. The third method involves increasing the number of shades of each color of ink used in the printer.
The present invention generally relates to a novel method and means for achieving variable color levels in ink printing, in particular acoustic ink printing, by using a lightening or darkening agent in combination with an underlying colored ink in a dot-on-dot type printing application.